The implantable defibrillator requires the use of electrodes to conduct large currents through the human heart. Previously, these electrodes have typically been two or more patches stitched to the heart. These are referred to as epicardial-patch electrodes and require the surgeon to open the chest cavity for placement.
To avoid the surgery required for the epicardial patches, electrode coils are sometimes inserted into the heart chambers via venous routes using various procedures known within the medical and surgical arts. In practice, a vein such as the right or left cephalic vein or the internal or external jugular vein is selected for insertion. The selected vein is exposed by a standard surgical cutdown procedure. The tapered end of a vein pick is inserted into the incised, selected vein. An introducer is then pushed underneath the vein pick into the vein. The defibrillation catheter is then slid into the introducer and the introducer, with the inserted catheter lead, is then gently fed into the vein. The introducer is then peeled away and discarded. Usually using fluoroscopy, the catheter is then gently pushed through the vein into the vena cava and thence into the heart, where it is positioned.
Defibrillation coils, used to deliver the electrical current to defibrillate the heart muscle, are often mounted or integral with other functional elements in defibrillation catheters. These coils are known as transvenous electrodes. One coil generally sits just above the right atrium (RA) in a location within the superior vena cava right (SVC). The other coil generally lies within the right ventricle at the right ventricular apex (RVA).
Unfortunately, catheter electrodes are often unable to direct sufficient current through enough of the heart muscle to effect defibrillation. For this reason, a small patch is often inserted just under the skin on the patient's left side, thus requiring additional, but minimal, surgery. This "subcutaneous patch" is not in direct contact with the heart, but provides for a current vector between the transvenous electrode to the subcutaneous patch, thereby passing through the heart muscle. Thus, the subcutaneous patch assists in directing current through the heart muscle, and hence, facilitates defibrillating the heart.
There are two primary electrical requirements for defibrillation electrodes. The first is that the resistance be low enough to allow the passage of a large current through the heart. The second requirement is that the current vector passes through the vast majority of the heart muscle. This second requirement is usually met by having sufficient extent to the electrodes and by careful positioning. Thus, other than placement, the primary barrier to increased utilization of catheter electrodes for defibrillation relates to the ability to successfully lower electrode resistances.
U.S. Pat. No. 5,265,623 issued to Kroll et al. attempted to more evenly distribute the electrical energy emitted by the claimed electrode during defibrillation by positioning the electrode-conductor connection at or near the mid-point of the electrode. Seeking better defibrillation efficacy by this design, Kroll et al. failed to realize the relative importance of relative electrode dimensions in reducing electrode resistances, other than stating that electrodes with smaller radii generated greater electrical fields. Kroll et al. also taught a preference for electrode lengths of less than 30 times the diameter of the electrode. While Kroll et al. represented an advance in defibrillating catheter technology, the importance of catheter electrode dimensions was neither known nor taught.
Another advance in this field was represented by the teachings of U.S. Pat. No. 5,257,634, issued to Kroll. Kroll correctly taught the importance of increased electrode length, but totally failed to address the lesser, but nonetheless substantial importance of catheter diameter. Kroll taught the construction and use of defibrillation electrodes that maximized the effective length of the electrode by employing flexible resilient extension members to the electrode body. While the electrodes of Kroll represented another advance in defibrillator cathode technology, the main purpose was to achieve the maximum possible effective electrode length without the appreciation of some optimum levels or ratios of catheter length and diameter design dimensions.
Detailed herein are new physical embodiments of cardioversion defibrillation transvenous catheters with lower electrode resistances. These improved catheters have been enabled by a theoretical discovery which nullifies previous assumptions that maximized electrode surface area or electrode diameter as the most efficient means of constructing catheters with minimum electrode resistances.